BUG/MINOR: server: do not use refcount in free_server in stopping mode

BUG/MINOR: server: do not use refcount in free_server in stopping mode

Currently there is a leak at process shutdown with dynamic servers with
check/agent-check activated. Check purges are not executed on process
stopping, so the server is not liberated due to its refcount.

The solution is simply to ignore the refcount on process stopping mode
and free the server on the first free_server invocation.

This should not be backported, unless dynamic server checks are
backported. In this case, the following commit must be backported first.
  7afa5c1843521ec3be7549592d2b38ccc9d68b73
  MINOR: global: define MODE_STOPPING